If you take a number of wounds greater than or equal to your Mighty, you're knocked unconscious and placed on death's door. Once you're at death's door, unless someone stabilizes you, you die at the end of the next round.
You may spend a fate point to automatically stabilize yourself when on death's door. This does not bring you back into the action, but you will stay alive until you're properly revived.
To stabilize someone on death's door, describe what you're doing to stabilize them and the GM will specify an attribute and skill for the Stabilize check. For example, if you're trying to bandage their wounds, you'll make a Survival check. The TN to successfully stabilize someone is 7 plus the number of wounds they currently suffer. This is called a Stabilize check.
Success means the victim is taken off death's door, but they won't regain consciousness until they have at least one hit point, either by way of magical healing or natural recovery. Failure means you either don't know how to stabilize the victim or you don't have the ability to do it: someone else can try, so long as they do it quickly!
Healing & Death's Door
Heroes on death’s door can’t be healed back to consciousness (magically or otherwise). For example the spell Heal only recovers HP for targets who are not on death’s door. The same is true of healing potions, or other effects that recover HP: such targets need to be stabilized and can only recover HP by resting once they are on death’s door, unless the spell or effect says otherwise. This is to prevent trivializing the danger of dying, and avoid comical situations where heroes pop up from near death as in a video game.
Critical Success & Failure
Critical success and critical failure are possible with a stabilize check, and how the results are resolved is open to interpretation by the GM. Usually critical failure means you can't be stabilized and your wounds are too severe to heal by any means; critical success might mean that not only did they stabilize you, but your wounds are reduced by 1, or maybe you're temporarily brought back into the action, despite that you have more wounds than Mighty.